1. Field of the Invention
This invention relates to films useful as encapsulants for photovoltaic (solar) cell modules. This invention particularly relates to films comprising ethylene acid copolymer ionomers useful as encapsulants for photovoltaic cell modules.
2. Description of the Related Art
Photovoltaic (solar) cell modules are units that convert light energy into electrical energy that can be useful for powering machinery, electrical equipment and appliances, for example. Typical or conventional construction of a solar cell module consists of 5 (five) structural layers. The layers of a conventional solar cell module are constructed in the following order starting from the top, or incident layer (that is, the layer first contacted by incident light) and continuing to the backing (the layer furthest removed from the incident layer): (1) incident layer/(2) encapsulant layer/(3) voltage-generating layer/(4) second encapsulant layer/(5) backing.
The function of the incident layer is to provide a transparent protective window that will allow sunlight into the cell module. The incident layer is typically a glass plate, but could conceivably be any material which is transparent to sunlight.
The encapsulating (encapsulant) layer is designed to encapsulate and protect the fragile voltage generating layer. Typically the encapsulant layer comprises two polymeric layers sandwiched around the voltage generating layer. The two encapsulant layers can be the same material or different and distinct materials. For expediency, unless specifically noted, when the encapsulant layers are mentioned hereinafter the singular form will be used and be taken to mean either one encapsulant layer or multiple encapsulant layers. However, the optical properties of at least the first encapsulant layer must be such that light can be effectively transmitted to the voltage-generating layer.
A conventional encapsulant layer typically comprises ethylene/vinyl acetate copolymer (EVA). Some encapsulant layers are obtained from a copolymer obtained by copolymerization of monomers of ethylene with monomers of ethylenically unsaturated carboxylic acids, wherein the copolymer is subsequently fully or partially neutralized to the acid salt. Neutralized ethylene/acid copolymers of this type are conventionally known as ionomers. Ionomers are commercially available from E.I. DuPont de Nemours and Company (DuPont). Still other materials can be used as an encapsulant layer.
The voltage-generating layer is typically a silicon wafer that converts photons of sunlight into electrical energy.
A solar cell backing functions to protect the solar cell module from the deterious effects of the environment. The requirements for a solar cell backing are: (1) good weatherability (that is, resistance to the effects of weather); (2) high dielectric strength; (3) low moisture vapor transmission rate (MVTR); and (4) mechanical strength. The backing layer must also have good adhesion to the second encapsulant layer, to prevent delamination of the cell materials.
In a solar cell module, high modulus in an encapsulant layer can be undesirable because stresses can be generated in the photovoltaic module which result in spalling, failure at the interface of the glass-top layer, or adhesive failure of the module. It has been found that in a solar cell module wherein the top layer is glass and the backing is a polymeric material and wherein the encapsulant layer is an ionomeric material, the modulus of the ionomer layer can be too high. High modulus in a commercially available material can sometimes be overlooked because the ionomer offers optical properties that are desirable for the application. Unfortunately, substitution for some materials having acceptable optical properties but less than acceptable modulus with commercially available ionomers having acceptably low modulus can be a poor alternative because the optical properties can be less than desirable.
Good optical properties are important in a solar cell module because it is a requirement of good performance that light incident to the cell be transmitted efficiently and effectively to the voltage-generated layer. Poor light transmission reduces the efficiency and/or productivity of the photovoltaic generation process.
Blending ionomers having acceptable modulus with other ionomers having acceptable optical properties can result in optical properties and modulus that are intermediate between the properties of the components. This can be an unsatisfactory result where improved optical properties or modulus are desirable.
It can be desirable, in the manufacture of solar cell modules, to provide an ionomer encapsulant layer that incorporates both the physical and optical properties desirable in a solar cell module.